Electrical devices such as displays, touch screens, heaters, bus bars, light sources, may contain a substrate provided with an indium tin oxide (ITO) layer as a transparent electrode. The coating of ITO is carried out by vacuum sputtering methods, which involve high substrate temperature conditions up to 250° C., and therefore, glass substrates are generally used. The high cost of the fabrication methods and the low flexibility of such electrodes, due to the brittleness of the inorganic ITO layer as well as the glass substrate, limit the range of potential applications. As a result, there is a growing interest in making all-organic devices, comprising plastic resins as a flexible substrate and carbon nanotube or organic electroconductive polymer layers as an electrode. Such plastic electronics allow low cost devices with new properties. Flexible plastic substrates can be provided with an electroconductive polymer layer by continuous hopper or roller coating methods (compared to batch process such as sputtering) and the resulting organic electrodes enable the “roll to roll” fabrication of electronic devices which are more flexible, lower cost, and lower weight.
Touch screens (also referred to as touch panels or touch switches) are widely used in conventional CRTs and in flat-panel display devices in computers and in particular with portable computers. FIG. 1 shows a typical resistive-type touchscreen 100 comprising a first electrode 120 that is on the side of the touchscreen that is nearer to the device that is referred herein below as the device side electrode and a second electrode 110 that is on the side of the touchscreen that is nearer to the user that is referred herein below as the touch side electrode. Device side electrode 120 comprises a transparent substrate, having a first conductive layer. Touch side electrode 110 comprises a transparent support, that is typically a flexible transparent support, and a second conductive layer that is physically separated from the first conductive layer by dielectric (insulating) spacer elements 30. The transparent substrate and support may be bonded together at their perimeter by adhesive 40 to make an assembly. The conductive layers have a sheet resistance selected to optimize power usage and position sensing accuracy. A voltage is developed across each of the conductive layers in turn by a controller (not shown). Deformation of the touch side electrode 110 by an external object such as a finger or stylus causes the second conductive layer to make electrical contact with first conductive layer, thereby transferring a voltage between the conductive layers. The magnitude of this voltage is measured by the controller through connectors 143, 144, 253, 254, connected via conductive patterns 145, 146, 255, 256, and metal bus bars 141, 142, 251, 252 formed on the edges of conductive layers, to locate the position of the deforming object.
FIG. 2 shows a 4-wire resistive touch screen configuration where the touch side 110 and device side 120 layers have been offset for clarity. The touch side layer has vertically arranged bus bars 141 and 142 which may be used to develop a voltage gradient in the horizontal direction, increasing in potential from left to right, by grounding the left bus bar 142 and raising the right bus bar 141 to the supply voltage (V+). Switching devices 301, shown as field effect transistors (FETs), are provided to selectively impose the voltage gradient or allow the bus bar potential to float, as determined by the gate level of the FET. Other switching devices, such as bipolar transistors, integrated circuit multiplexers, application specific integrated circuits (ASICs), relay contacts, and other devices known to a practitioner schooled in electronic circuit design may be used to accomplish the equivalent function.
A similar arrangement of horizontal bus bars 251 and 252 on the device side layer 120 along with switching devices 302, allow for a vertical voltage gradient, increasing in potential from top to bottom, to be created in that layer when the top electrode is grounded and the lower bus is raised to the (V+) potential via their respective switch devices.
The direct current power supply, switching devices, voltage measuring subsystems (not shown), and communication subsystems (not shown), in combination form a “controller” for the touch screen. The touch screen controller may also incorporate a microcontroller, microprocessor, analog multiplexer (MUX), analog to digital converter (ADC), digital signal processor (DSP) or other digital logic to control the various subsystems and coordinate the touch position measuring process.
The horizontal component of the two-dimension touch position is determined by applying logic levels to terminals “A”, to place those switching devices in a conductive state, thus imposing the horizontal voltage gradient in the touch side electrode. The logic levels on terminals “B” place those switches in an off state, allowing the voltage of the device side electrode to be pulled to the local voltage at the point where it makes contact with the touch side electrode. The potential of the device side layer may be measured at either horizontal bus bar (terminal Sh 254 shown).
The vertical component of touch position is determined by applying logic levels to turn switches “A” in an off state and switches “B” in an on state, creating a vertical voltage gradient in the device side conductor. The voltage at the touch side electrode is then pulled to the local potential at the point of contact with the device side electrode and may be sensed at either vertical bus bar (terminal Sv 144 shown).
The 4-wire touch screen measurement may be improved by providing independent sensing traces (not shown) between each bus bar and the voltage measurement subsystem. This 8-wire configuration makes advantageous use of the high impedance of the voltage measurement subsystem, and thus low current in the additional sense traces, to reduce errors associated with voltage drops in the 4-lead leads of the earlier discussed configuration.
FIG. 3 shows an alternative 5-wire touch screen architecture wherein the rectangular device side conductor 120 is provided with combined driving and sensing terminations at each of the four corners. One corner 251 (upper left shown) is connected to the positive power supply while the diagonally opposite corner is grounded. The remaining corners are connected via switching devices to either V+ or ground according to logic levels generated by the touch screen controller.
The horizontal component of the two-dimension touch position is determined by applying logic levels to terminals “A”, to place switching devices 301 in a conductive state, thus imposing a horizontal voltage gradient in the touch side electrode 120. The logic levels on terminals “B” place those switches 302 in an off state.
The touch side conductor plane 110 forms the fifth terminal of the 5-wire configuration and is used to sense the local voltage of the device side conductor at the point of contact when touched via terminal “S”.
The vertical component of touch position is determined by applying logic levels to turn switches “A” to an off state and switches “B” to an on state, creating a vertical voltage gradient in the device side conductor. The voltage at the touch side electrode is then pulled to the local potential at the point of contact with the device side electrode and may be sensed at terminal “S”.
The 5-wire configuration may be enhanced by the addition of separate sensing connections (not shown) to the device side conductor at the corners where the drive voltage is selectively switched. In this 7-wire configuration, the additional traces may be used to sense the actual device side voltage thus reducing the errors associated with voltage drops across the switching devices or current carrying connecting traces.
In any of the touch screen configurations described, the roles and terminations of the touch side and device side conductors may be interchanged as desired without effecting the function or operating principle of the touch screen. Further, the designations of horizontal, vertical, left, right are arbitrary and are used for illustrative purposes. The direction of the voltage gradient in each sensing mode is chosen by the designer to suit the preferred orientation of the installed device. However, in the prior art, it is only necessary and possible to impose the voltage gradient within the selected planar conductor in a single direction for each sense mode.
Welsh et al in U.S. Pat. No. 6,469,267 illustrate the application of reduced DC voltage potential across a series circuit comprised of a 1 kOhm sense resistor, 9 kOhm dropping resistor, touch side conductor comprised of intrinsically conductive polymer and, via an intermittent contact patch created by repeatedly deforming the touch side conductor, to a device side conductor of ITO. The use of limited DC voltage resulted in an increased number of make and break cycles before the contact resistance reached a failure level, as determined by a 50% reduction in the series current through the sense resistor. The disclosure is applicable to the touch detection event, where current across the conductor interface is appreciable and may be limited by either increased resistance or decreased voltage as predicted by Ohms law.
U.S. Pat. No. 6,469,267 further illustrates the application of 60 Hz sinusoidal alternating current (AC) drive as a means of increasing the number of actuations of the switch like device before failure.
Thus, there is a new need to provide improved controller architectures that mitigate the degradation that results from using prior art control methodologies with organic conductor layers. An improved controller should operate equally well over a range of drive voltage and not depend on continuously time varying sinusoidal voltage levels. For resistive touch screen applications, it is advantageous for the improved controller to be backward compatible with preexisting inorganic touch screens.